Dynamic Measures of Low Back Performance.

A variety of methods for analyzing dynamic motor performance in the low back were outlined. Coordinate systems for the subject and any external objects involved in the test were discussed, as well as the importance of expressing careful details about the movement to the human subject. Possible experimental variables included independent, dependent, controlled, and confounding variables. Current human performance measures were viewed as a continuum, with isometric strength at one end and free dynamic exertion at the other. Isometric assessment involved keeping the muscle length constant. Outputs of this test were force level, endurance, or successful repetition. Isokinetic assessment dealt with keeping the body or object velocity constant and possible outputs of this study included force, torque and number of repetitions. Isoacceleration, isojerk, isoinertial, and isoforce testing were also summarized. When isometric and isoforce tests were combined, the subject held the weight motionless. In free dynamic assessment the subject was allowed to move freely. External forces, measured at the subject device interface, as well as internal forces were considered, since both affected the subject's performance. While internal forces were not directly measurable, several techniques were discussed that measure internal forces indirectly, such as electromyography, intraabdominal pressure, and intradiscal pressure. These three methods were considered both invasive and expensive, and the last, intradiscal pressure was not legal in the United States. The authors conclude that a trade off exists when conducting such tests since, as the realism of the test increases, knowledge about the internal forces decreases. Although measuring dynamic motor performance is not an easy task, more simple, direct ways of measuring internal forces will develop as the technology improves.